The present invention relates generally to a lithography system. More particularly, the present invention relates to improved techniques for supporting and holding a reticle.
Lithography systems used in the manufacture of integrated circuits and related devices have been around for some time. Such systems have proven extremely effective in the precise manufacturing and formation of very small details in the product. In most lithography systems, a circuit image is written on a wafer by projecting a beam through a patterned reticle. By way of example, optical lithography systems and electron beam projection systems, which tend to produce finer geometries than optical lithography systems, have been widely used to reproduce a circuit image on a silicon wafer. In optical lithography systems, a beam of light is used to scan the surface of the reticle. In electron beam projection lithography systems, a beam of electrons is used to scan the surface of the reticle.
Electron beam projection lithography systems typically include an illuminator for directing electron beams of finite area through patterns resident on the surface of a reticle, a stage for moving the reticle relative to the beam, a chuck for supporting the reticle relative to the stage, and a projector for projecting the transmitted electron beam (e.g., the pattern of electrons passing through the reticle) onto the surface of a wafer. In order to process the wafer, the stage is moved along a linear scan path while the electron beams are swept orthogonally to the linear path so that all or any selected part of the patterned reticle is scanned. Although only a small portion of the reticle is imaged at any one time, the surface of the reticle is sequentially exposed to electron beams, allowing a pattern to be built up on the wafer.
In general, the beam sweep is in a direction parallel to the Y-axis as viewed at the reticle, and the linear scan path is in a direction parallel to the X-axis as viewed at the reticle. More particularly, as the beams are swept, the stage carrying the reticle is typically moved back and forth in the X direction while being incremented in the Y-direction at the end of each traversal so that the beam sweeps along a substantially serpentine path across a predetermined area of the reticle. The predetermined area may correspond to a single identified sub area, a plurality of identified sub areas or the entire reticle. Furthermore, the chuck is required to hold the reticle in place while the stage is moved. In most cases, a large clamping force is needed to overcome the forces generated by the high acceleration (e.g., 4 g) associated with the stage. If the holding force is not sufficient, then the reticle may peel away or shift from the chuck during high accelerations.
Electron beam projection lithography systems generally require precise tolerances in order to achieve finer geometries. For example, because an electron beam projection lithography system generally determines reticle position relative to the stage position, the system must be capable of precisely locating the reticle relative to the stage. In general, the reticle must effectively not be allowed to change position with respect to the stage. As should be appreciated, reticle misalignment tends to cause errors in projecting the reticle pattern onto the wafer surface, especially with the extreme level of accuracy that is sought in electron beam projection lithography.
Unfortunately, reticle misalignment can be encountered when external stresses are induced on the reticle or other related structures such as the chuck or stage. Such stresses may be caused by mechanical distortion of the reticle or chuck to which the reticle is mounted, or by differential thermal expansion or contraction between the reticle and the chuck. With regards to differential thermal expansion or contraction, stresses may be transmitted to the reticle when the reticle expands or contracts while the chuck remains static or when the chuck expands or contracts while the reticle remains static. In most cases, a high intensity electron beam tends to raise the temperature of the reticle during processing thus making the reticle expand. By way of example, the reticle may bow if the chuck tries to keep its original dimension while the reticle is trying to expand. Alternatively, if the stress is too high then the reticle may slip from its original position. This change in height or position may adversely affect the projected pattern. Moreover, even if the chuck complied to the expansion and contraction of the reticle, stresses may be transmitted to the reticle and/or chuck when the reticle expands or contracts and the supporting structure remains static.
Furthermore, the throughput associated with electron beam projection systems has generally been limited, due at least in part to the fact that electron beam systems operate in a vacuum. Also, within electron beam projection systems, the implementation of a step and scan configuration may be difficult. Specifically, implementing a step and scan configuration with respect to a stage which scans reticles, e.g., a reticle stage, is difficult, as electron beam projection systems have specific requirements which are not requirements for typical optical lithography systems. By way of example, an electron beam projection system generally must operate in a high vacuum environment. Further, an electron beam projection system may not include moving magnets, as moving magnets cause the magnetic field associated with the electron beam projection system to change. An electron beam projection system also may not having moving iron structures, due to the fact that moving iron dynamically alters the static magnetic fields around an electron beam lens. Finally, an electron beam projection system may not have metal parts which move such that eddy currents are generated in static magnetic fields with concomitant additional varying magnetic fields.
Therefore, what is needed is a method and an apparatus for enabling reticles to be precisely and stablely held within an electron beam projection lithography system.
The invention relates, in one embodiment, to a lithography system for processing a substrate. The lithography system includes a stage for moving the substrate relative to a beam. The lithography system further includes a chuck for securely holding the substrate during stage movement. The lithography system additionally includes a support assembly for holding the chuck in a fixed position relative to the stage while accommodating for deformations in either the chuck or the stage during processing.
The invention relates, in another embodiment, to a support assembly for holding a chuck in a fixed position relative to a stage while allowing some plasticity of the chuck during processing. The support assembly includes a plurality of flexures, each of which has one end attached to the stage and an opposite end attached to the chuck. Additionally, the plurality of flexures work together to restrain the chuck from lateral, vertical and rotational movements while allowing some expansion or contraction of the chuck relative to the stage.